452 research outputs found
The Sparsest Clusters With O Stars
There is much debate on how high-mass star formation varies with environment,
and whether the sparsest star-forming environments are capable of forming
massive stars. To address this issue, we have observed eight apparently
isolated OB stars in the SMC using HST's Advanced Camera for Surveys. Five of
these objects appear as isolated stars, two of which are confirmed to be
runaways. The remaining three objects are found to exist in sparse clusters,
with <10 companion stars revealed, having masses of 1-4 solar mass. Stochastic
effects dominate in these sparse clusters, so we perform Monte Carlo
simulations to explore how our observations fit within the framework of
empirical, galactic cluster properties. We generate clusters using a simplistic
-2 power-law distribution for either the number of stars per cluster (N_*) or
cluster mass (M_cl). These clusters are then populated with stars randomly
chosen from a Kroupa IMF. We find that simulations with cluster lower-mass
limits of M_cl,lo >20 solar mass and N_*,lo >40 match best with observations of
SMC and Galactic OB star populations. We examine the mass ratio of the
second-most massive and most massive stars (m_max,2/m_max), finding that our
observations all exist below the 20th percentile of our simulated clusters.
However, all of our observed clusters lie within the parameter space spanned by
the simulated clusters, although some are in the lowest 5th percentile
frequency. These results suggest that clusters are built stochastically by
randomly sampling stars from a universal IMF with a fixed stellar upper-mass
limit. In particular, we see no evidence to suggest a m_max - M_cl relation.
Our results may be more consistent with core accretion models of star formation
than with competitive accretion models, and they are inconsistent with the
proposed steepening of the integrated galaxy IMF (IGIMF).Comment: 19 pages, 12 figures, accepted for publication in Ap
More Evidence on the Presence of an Unknown Toxic Substance(s) in the Sagabean
Saga (Adenanthera pavonina Linn) tergolong kacang-kacangan (Leguminosae), maka diduga seperti juga hampir semua kacang-kacangan, mengandung faktor-faktor "anti-nutrisi", seperti trypsin inhibitor, fitohaemagglutinin dan saponin. Telah diketahui bahwa ada beberapa kacang-kacangan, yang di samping faktor "anti-nutrisi", juga mengandung zat beracun seperti: koro wedus (Dolichos lablab) dan kratok (Phaseolus lunatus), yang mengandung sianida (HCN), lamtoro (Leucaena glauca) yang mengandung mimosine, dan saga (Abrus precatorius Linn) ycng mengandung racun abrin. Timbul pertanyaan apakah biji saga juga mengandung racun, di samping faktor anti-nutrisi tersebut di atas. Pada biji saga yang telah dikuliti, direndam, dicuci dan direbus dapat dianggap bahwa faktor-faktor anti-nutrisinya telah hilang. Tetapi walaupun demikian terbukti dengan percobaan tikus putih muda, bahwa biji saga yang telah diolah tetap hanya dimakan sedikit saja, juga bila ditambah dengan methionine dan threonine, dua asam amino yang sudah diketahui sangat terbatas kadarnya dalam protein biji saga. Dari percobaan ini dapat ditarik kesimpulan, bahwa dalam biji saga juga terdapat suatu zat toksik (racun) yang tidak dapat dihilangkan dengan cara pengolahan tersebut di atas
An anisotropic distribution of spin vectors in asteroid families
Current amount of ~500 asteroid models derived from the disk-integrated
photometry by the lightcurve inversion method allows us to study not only the
spin-vector properties of the whole population of MBAs, but also of several
individual collisional families. We create a data set of 152 asteroids that
were identified by the HCM method as members of ten collisional families, among
them are 31 newly derived unique models and 24 new models with well-constrained
pole-ecliptic latitudes of the spin axes. The remaining models are adopted from
the DAMIT database or the literature. We revise the preliminary family
membership identification by the HCM method according to several additional
criteria - taxonomic type, color, albedo, maximum Yarkovsky semi-major axis
drift and the consistency with the size-frequency distribution of each family,
and consequently we remove interlopers. We then present the spin-vector
distributions for eight asteroidal families. We use a combined orbital- and
spin-evolution model to explain the observed spin-vector properties of objects
among collisional families. In general, we observe for studied families similar
trends in the (a_p, \beta) space: (i) larger asteroids are situated in the
proximity of the center of the family; (ii) asteroids with \beta>0{\deg} are
usually found to the right from the family center; (iii) on the other hand,
asteroids with \beta<0{\deg} to the left from the center; (iv) majority of
asteroids have large pole-ecliptic latitudes (|\beta|\gtrsim 30{\deg}); and
finally (v) some families have a statistically significant excess of asteroids
with \beta>0{\deg} or \beta<0{\deg}. Our numerical simulation of the long-term
evolution of a collisional family is capable of reproducing well the observed
spin-vector properties. Using this simulation, we also independently constrain
the age of families Flora (1.0\pm0.5 Gyr) and Koronis (2.5-4 Gyr).Comment: Accepted for publication in A&A (September 16, 2013
Massive Field Stars and the Stellar Clustering Law
The distribution of N*, the number of OB stars per association or cluster,
appears to follow a universal power-law form in the local Universe.
We evaluate the distribution of N* in the Small Magellanic Cloud using recent
broadband optical and space-ultraviolet data, with special attention to the
lowest values of N*. We find that the power-law distribution in N* continues
smoothly down to N*=1. This strongly suggests that the formation of field
massive stars is a continuous process with those in associations, and that the
field stars do not originate from a different star formation mode. Our results
are consistent with the model that field massive stars represent the most
massive members in groups of smaller stars, as expected if the clustering law
applies to much lower masses as is expected from the stellar initial mass
function (IMF). These results are consistent with the simultaneous existence of
a universal IMF and a universal clustering law. Jointly, these laws imply that
the fraction of field OB stars typically ranges from about 35% to 7% for most
astrophysical situations, with an inverse logarithmic dependence on the most
populous cluster, and hence, on galaxy size and/or star formation rate. There
are important consequences for global feedback effects in galaxies: field stars
should therefore contribute proportionately to the volume of the warm ionized
medium, and equal relative contributions by superbubbles of all sizes to the
interstellar porosity are expected.Comment: Accepted by AJ. 13 pages, 9 figures in 11 files, uses emulateapj.st
Past and present star formation in the SMC: NGC 346 and its neighborhood
In the quest of understanding how star formation occurs and propagates in the
low metallicity environment of the Small Magellanic Cloud (SMC), we acquired
deep F555W (~V), and F814W (~I) HST/ACS images of the young and massive star
forming region NGC 346. These images and their photometric analysis provide us
with a snapshot of the star formation history of the region. We find evidence
for star formation extending from ~10 Gyr in the past until ~150 Myr in the
field of the SMC. The youngest stellar population (~3 +/- 1 Myr) is associated
with the NGC 346 cluster. It includes a rich component of low mass pre-main
sequence stars mainly concentrated in a number of sub-clusters, spatially co-
located with CO clumps previously detected by Rubio et al. (2000). Within our
analysis uncertainties, these sub-clusters appear coeval with each other. The
most massive stars appear concentrated in the central sub-clusters, indicating
possible mass segregation. A number of embedded clusters are also observed.
This finding, combined with the overall wealth of dust and gas, could imply
that star formation is still active. An intermediate age star cluster, BS90,
formed ~4.3 +/-0.1 Gyr ago, is also present in the region. Thus, this region of
the SMC has supported star formation with varying levels of intensity over much
of the cosmic time.Comment: 38 pages, 13 figures, 3 tables; AJ accepte
A Fractal Analysis of the HI Emission from the Large Magellanic Cloud
A composite map of HI in the LMC using the ATCA interferometer and the Parkes
multibeam telescope was analyzed in several ways in an attempt to characterize
the structure of the neutral gas and to find an origin for it. Fourier
transform power spectra in 1D, 2D, and in the azimuthal direction were found to
be approximate power laws over 2 decades in length. Delta-variance methods also
showed the same power-law structure. Detailed models of these data were made
using line-of-sight integrals over fractals that are analogous to those
generated by simulations of turbulence with and without phase transitions. The
results suggested a way to measure directly for the first time the
line-of-sight thickness of the cool component of the HI disk of a nearly
face-on galaxy. The signature of this thickness was found to be present in all
of the measured power spectra.
The character of the HI structure in the LMC was also viewed by comparing
positive and negative images of the integrated emission. The geometric
structure of the high-emission regions was found to be filamentary, whereas the
geometric structure of the low-emission (intercloud) regions was found to be
patchy and round. This result suggests that compressive events formed the
high-emission regions, and expansion events, whether from explosions or
turbulence, formed the low-emission regions. The character of the structure was
also investigated as a function of scale using unsharp masks.
All of these results suggest that most of the ISM in the LMC is fractal,
presumably the result of pervasive turbulence, self-gravity, and self-similar
stirring.Comment: 30 pages, 21 figures, scheduled for ApJ Vol 548n1, Feb 10, 200
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